Housings for switching appliances, in particular for command and signal units, generally consist of plastics material and are composed of an upper and a lower part which are rigidly interconnected by fixing screws.
FIG. 3 shows an example of a switching appliance housing 100 composed of an upper part 102 having openings 103 for operating and switching buttons and a lower part 104. Four screws 106 having lens countersunk heads and plastics material threads fix the upper part 102 to the lower part 104. To achieve as high a level of protection as possible, a seal 108 may be provided between the upper and the lower part. In this case, the sealing material sits in the upper part 102 and the shape thereof extends below the countersunk head of each of the screws 106, in such a way that when the screws 106 are tightened the seal 108 seals the screws 106 off from the housing.
Because the screw head sits somewhat lowered in the counterbore in the housing upper part 102, and is additionally somewhat smaller than the counterbore depression of the upper part 102 due to the required tolerance of the head, there is a gap, in which deposits such as dirt, cleaning agents or even bacteria may collect, between the screw head and the upper housing part. Likewise, deposits of this type may collect in the slits serving as a tool interface in screw heads.
In particular if switching appliance housings of this type are used in hygiene areas, for example in production plants for pharmaceuticals or foodstuffs, deposits of this type are a serious problem, in particular if they are released and enter processed foodstuffs or pharmaceuticals. The cleaning of gaps or depressions of this type in housings is also problematic.
Therefore, gaps and depressions in housings such as switching appliance housings generally constitute a constant risk of contamination in hygiene areas for the products manufactured under hygiene rules.
German laid-open publication DE 10 2009 044 746 A1 describes a screw comprising a cylindrical portion or shaft arranged between a screw head and a thread portion, having a greater diameter than the thread portion, and having approximately centrally a groove for receiving a seal.
US laid-open publication US 2007/0098520 A1 describes a bolt for impressing in a metal sheet, which has a groove arranged between the lower face of the bolt head and an annular projection. For sealing a hole into which the bolt is to be inserted, the bolt head is coated with a sealing material such as epoxy resin. Recesses may also be provided on the lower face of the bolt head for receiving sealing material.
German utility model specification DE 20 2007 014 325 U1 describes a mounting arrangement comprising a resilient ring, for example an O-ring, which is inserted into an annular groove that is introduced into the shaft of a screw of the arrangement approximately centrally in the longitudinal extension of the shaft.